Xenobiotic nuclear receptors, PXR and CAR, are activated by numerous chemicals and induce the Phase I, II and III clearance pathways through their association with specific transcriptional coactivators. Although some environmental contaminants including those found in the Superfund sites are known to activate PXR, CAR or both in a species-specific manner, the effects of the receptor activation on the toxicity are not clear. The studies proposed in this project are aimed at defining, in molecular terms, how the receptors and their xenobiotic and endobiotic ligands influence physiology by controlling gene expression patterns. The hypothesis is that certain toxic environmental agents activate either PXR and/or CAR, leading to both acute and chronic perturbations of the enterohepatic clearance system, production of supertoxic metabolites and endocrine disruption. To test these ideas, the investigators will combine genomic, molecular genetic, and biochemical approaches to 1) identify environmental toxins that activate PXR and/or CAR using a high-throughput cell-based reporter systems and an in vitro coactivator recruitment assay; 2) create a transgenic mouse line that replaces endogenous PXR with recombinant human PXR in both the liver and intestine to humanize the xenobiotic response over the entire enterohepatic system; 3) determine the contribution of PXR and CAR towards the long-term and acute toxicity of chemicals using knockout and transgenic humanized animals; and 4) identify PXR and CAR target genes that contribute to the toxicogenomic response using DNA oligonucleotide microarray analysis. Taken together, the proposed experiments will uncover details of the molecular mechanisms by which environmental substances found at Superfund sites exert adverse effects and will also provide unique and quantitative tools to predict toxicity.